NGC 2818 is often heralded as one of the Galaxy's few planetary nebulae to be discovered as a member of an open star cluster. The other celebrated case is the planetary nebula NGC 2438 in the open star cluster designated Messier 46.
Recent investigations, however, suggest that both cases merely amount to a chance alignment, as the objects are actually located at varying distances along the line-of-sight. To date, there has yet to be a single established case of a Galactic planetary nebula discovered in an open cluster.
Planetary nebulae have been detected in several globular star clusters in our Galaxy. These densely-packed, gravitationally-bound groups of 100,000s to millions of stars are far older than their open cluster counterparts.
Image : NASA, ESA, J. Hester and A. Loll (Arizona State University)